My brother in laws cat fell ill last week. He had a lot of fluid in his lungs, and died before anything could be done. Their vet said it was feline leukemia, but everything I have read says that f.l. is not a sudden onset type disease, but a slow, chronic disease that can lead to cancer. If this is true, what could have killed the kitty? He has 2 other kitties that aren't sick, but being tested for F.L.